Hong Kong quarantine rules pre Christmas required returning boarders to spend 14 days in quarantine. That is what they did. Not sure where your 22 days and 21 days came from but certainly wasn’t in force when the Boarding Schools broke up. And Hong Kong borders are tight, wrist bands, secure quarantine, testing etc. so whatever the regulation were then they were tightly enforced, in contrast to the U.K.
My information comes from living in HK
but I think we're slightly at cross purposes, as I agree that at end of winter term, the inbound quarantine was 14 days.
My comment was based on the fact that from observation a lot of UK boarders didn't come back to HK at Christmas because there was already talk of a 21 day quarantine and because even 14 days was prohibitively long when some schools also wanted students to quarantine in UK prior to the Lent term starting. Also, HK was basically shut down at Christmas so no fun to be had here. A lot stayed in UK, presumably with friends or relatives. Therefore, they were then "stuck" when schools didn't reopen in Jan as expected (whether that expectation is reasonable is a whole other thread). Also, a lot of UK boarding schools wouldn't commit to providing online learning if they were open so if parents chose to bring children home at Christmas and keep them here for the Jan-Mar term, they would just have missed that term of school . Some people did therefore send their DC back early Jan.
The only way anyone got back to HK from UK after 25 December was by doing the "3+3". There was no other way, other than a couple of repatriation flights in April for the truly desperate (mandatory quarantine hotel selected is awful- you'd rather be in the government quarantine centre). There is talk that direct flights may resume mid May.
I imagine with hindsight, many parents wish they'd done it differently, but hindsight is an amazing thing. HK schools here have been closed since Jan 2020 with a few patches of part time, so if a parent had chosen to bring the child back, they'd have been putting them into an unfamiliar school and unfamiliar curriculum, 95% online, in possibly a critical year, that's if you could find a school to accept the child (Hk local private and good state schools are hugely competitive, you get in for Year 7 and you stay put- v little movement). For myself, I wish I'd come back to UK with the DC for this academic year because even losing 2 months, my DC would have had a better year overall- as it is they've had 40% of the usual in person time (assuming schools stay open until the summer holidays) and there's a lot of plexiglass screens.
I guess what I'm saying is people just had to make decisions under pressure based on what might or might not happen. There are a lot of HK specific factors that currently impact why parents might be keen to keep the BS thing going.